1. Field of the Invention
The present invention relates to low dielectric constant nanoporous silica and to improved processes for producing the same on substrates suitable for use in the production of integrated circuits.
2. Description of the Prior Art
As feature sizes in integrated circuits approach 0.25 xcexcm and below, problems with interconnect RC delay, power consumption and signal cross-talk have become increasingly difficult to resolve. It is believed that the integration of low dielectric constant materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications will help to solve these problems.
Nanoporous Films
One material with a low dielectric constant is nanoporous silica, which, as a consequence of the introduction of air, which has a dielectric constant of 1, into the material via its nanometer-scale pore structure, can be prepared with relatively low dielectric constants (xe2x80x9ck). Nanoporous silica is attractive because it employs similar precursors, including organic-substituted silanes, e.g., tetraethoxysilane (xe2x80x9cTEOSxe2x80x9d), as are used for the currently employed spin-on-glasses (xe2x80x9cSOGxe2x80x9d) and chemical vapor disposition (xe2x80x9cCVDxe2x80x9d) silica SiO2. Nanoporous silica is also attractive because it is possible to control the pore size, and hence the density, material strength and dielectric constant of the resulting film material. In addition to a low k, nanoporous silica offers other advantages including: 1) thermal stability to 900xc2x0 C., 2) substantially small pore size, i.e, at least an order of magnitude smaller in scale than the microelectronic features of the integrated circuit), 3) as noted above, preparation from materials such as silica and TEOS that are widely used in semiconductors, 4) the ability to xe2x80x9ctunexe2x80x9d the dielectric constant of nanoporous silica over a wide range, and 5) deposition of a nanoporous film can be achieved using tools similar to those employed for conventional SOG processing.
Thus, high porosity in silica materials leads to a lower dielectric constant than would otherwise be available from the same materials in nonporous form. In an additional advantage for nanoporous silica, additional compositions and processes may be employed in nanoporous silica, relative to a denser material. Other materials requirements include the need to have all pores substantially smaller than circuit feature sizes, the need to manage the strength decrease associated with porosity, and the role of surface chemistry on dielectric constant and environmental stability.
Density (or the inverse, porosity) is the key parameter of nanoporous silica that controls the dielectric constant of the material and this property is readily varied over a continuous spectrum from the extremes of an air gap at a porosity of 100% to a dense silica with a porosity of 0%. As density increases, dielectric constant and mechanical strength increase but the pore size decreases and vice versa. This suggests that the density range of nanoporous silica must be optimally balanced between for the desired range of low dielectric constant, and the mechanical properties acceptable for the desired application.
Nanoporous silica films have previously been fabricated by a number of methods, without achieving significant practical or commercial success. For example, nanoporous silica films have been prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate, e.g., a silicon wafer suitable for producing an integrated circuit, by conventional methods, e.g., including spin-coating and dip-coating. The substrate optionally has raised lines on its surface and preferably has electronic elements and/or electrical conduction pathways incorporated on or within its surface. The as-spun film is typically catalyzed with an acid or base catalyst and additional water to cause polymerization/gelation (xe2x80x9cagingxe2x80x9d) and to yield sufficient strength so that the film does not shrink significantly during drying.
Another previously proposed method for providing nanoporous silica films was based on the observation that film thickness and density/dielectric constant can be independently controlled by using a mixture of two solvents with dramatically different volatility. The more volatile solvent evaporates during and immediately after precursor deposition. The silica precursor, typically partially hydrolyzed and condensed oligomers of TEOS, is applied to a suitable substrate and polymerized by chemical and/or thermal means until it forms a gel. The second solvent, called the Pore Control Solvent (PCS) is usually then removed by increasing the temperature until the film is dry. The second solvent is then removed by increasing the temperature. Assuming that no shrinkage occurs after gelation, the density/dielectric constant of the final film is fixed by the volume ratio of low volatility solvent to silica. EP patent application EP 0 775 669 A2, which is incorporated herein by reference, shows a method for producing a nanoporous silica film with uniform density throughout the film thickness.
Another method for producing nanoporous dielectrics is through the use of sol-gel techniques whereby a sol, which is a colloidal suspension of solid particles in a liquid, transforms into a gel due to growth and interconnection of the solid particles. One theory that has been advanced is that through continued reactions within the sol, one or more molecules within the sol may eventually reach macroscopic dimensions so that they form a solid network which extends substantially throughout the sol. At this point, called the gel point, the substance is said to be a gel. By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, the term xe2x80x9cgelxe2x80x9d as used herein means an open-pored solid structure enclosing a pore fluid.
Protecting the Surfaces of Nanometer Scale Pores
As the artisan will appreciate, a useful nanoporous film must meet a number of criteria, including having a dielectric constant (xe2x80x9ckxe2x80x9d) falling within the required value range, having a suitable thickness (xe2x80x9ctxe2x80x9d) (e.g., measured in angstroms), having an ability to effectively fill gaps on patterned wafers, and having an effective degree of hydrophobicity. If the film is not strong enough, despite achieving the other requirements, the pore structure may collapse, resulting in high material density and therefore an undesirably high dielectric constant. In addition, the surfaces of the resulting nano-scale pores carry silanol functional groups or moieties. Silanols, and the water that can be adsorbed onto the silanols, are highly polarizable and will raise the dielectric constant of the film. Thus, the requirement for hydrophobicity arises from the requirement for a reduced range of dielectric constant relative to previously employed materials.
Previous attempts to solve this problem and to provide hydrophobic nanoporous films free of silanols and adsorbed water have employed the process of silylation, which is the derivatization of a surface with a capping reagent, e.g., trimethylsilyl [TMS, (CH3)3SiOxe2x80x94]. However, previous silylation processes have not been successful in achieving the desired hydrophobic properties for nanoporous silica.
In one such failed method, the wet nanoporous silica film was subjected to the additional step of exposing the film to a liquid mixture of solvent and a surface modification agent suitable for silylating the pore surface, e.g., hexamethyldisilazane [HMDZ, (CH3)3SiNHSi(CH3)3]. The purpose of the solvent is to both carry the agent, e.g., HMDZ, inside the nano-scale pores and into the pore volume, as well as providing the additional advantage of lowering the surface tension of the pore fluid before drying, thus avoiding mechanical stresses on the pore structure. The aim was for the surface modification agent to render the surfaces of the nano-scale pore structures hydrophobic by capping the silanols in the film with a hydrophobic moiety, e.g., by means of the reactions illustrated by Equations 1 and 2:
2SiOH+R3SiNHSiR3xe2x86x922SiOSiR3+NH3xe2x80x83xe2x80x83Equation 1:
SiOH+R3SiOHxe2x86x92SiOSiR3+H2Oxe2x80x83xe2x80x83Equation 2:
wherein each xe2x80x9cRxe2x80x9d is independently selected and is, e.g., H, any alkyl, aryl, alkylaryl and so forth, e.g., methyl, ethyl, phenol and any other suitable art-known moiety, provided that the capped silanols, i.e, the SiOSiR3 moieties, provide a film with effective hydrophobicity while avoiding any significant undesirable changes to other film parameters. In a more specific example, silylation may be accomplished by employing the following reaction, shown by Equation 3 below:
(CH3)3SiNHSi(CH3)3+2SiOH⇄2xe2x89xa1SiOSi(CH3)3+NH3xe2x80x83xe2x80x83Equation 3:
By capping the silanols so that the surface of the dried nanoporous silica is rendered hydrophobic, it was intended that the silanols be removed and that the adsorption of atmospheric moisture into the porous surface be prevented, thereby providing a lower and more stable range for the dielectric constant of the resulting nanoporous film product.
However, this process suffers from a number of difficulties. For example, when the capping reaction has been conducted on the gelled film using previously available liquid reagents, the process has required a second coater (or halved throughput on a single coater track) and a large excess of solvent/surface modification agent, all of which generates significant waste. In addition, this previous process is known to increase the impurity level in the final film product and fails to provide surfaces with a sufficient degree of silylation, resulting in a nanoporous film that is not sufficiently hydrophobic to prevent undesirable increases in the dielectric constant caused by adsorbed atmospheric moisture.
Other previous attempts to silylate pore surfaces of nanoporous films have included, conducting the silylation reaction before deposition of the film onto a substrate, but that has resulted in films of lower mechanical strength and reduced dielectric constant stability in the presence of atmospheric moisture. Since post-deposition surface modification yields the best film properties, that approach is preferred.
Thus, there remains a need in the art for new processes which eliminate all of the aforementioned problems, as well as overcoming other obstacles to the use of nanoporous silica in the fabrication of integrated circuits.
In order to solve the above mentioned problems and to provide other improvements, the invention provides new methods for effectively silylating nanoporous silica films to a desired range of dielectric constant significantly lower than has previously been obtained. The improved silylation processes provided herein allow for production of nanoporous silica films that are sufficiently hydrophobic to avoid moisture adsorption and to provide dielectric constant values that are both low and stable, while retaining other desirable characteristics required in the production of integrated circuits.
Thus, in one embodiment, the invention provides for an improved vapor-phase silylation process wherein the surface modification agent, e.g., silylation agent, is employed in a vapor-phase and optionally with a co-reactant and/or co-solvent. In another embodiment, the invention provides for an improved dual phase liquid-based silylation process wherein the surface modification agent, e.g., silylation agent, is employed in solution in combination with a ketone co-solvent.
The vapor phase and solution based processes according to the invention are conducted by treating a suitable nanoporous silica film, that has a pore structure with hydrophilic pore surfaces, and that is present on a substrate. The nanoporous silica film is optionally prepared on the substrate immediately prior to the time of treatment, or may be pre-prepared and stored or obtained from another source. It should also be mentioned that the nanoporous silica films to be treated by the vapor phase or solution based processes of the invention are optionally aged before or after conducting the inventive treatment, but preferably the film is aged prior to conducting the silylation reaction.
The vapor phase processes are conducted by reacting the nanoporous silica film with a vapor phase material that includes an effective amount of a surface modification agent and optionally a co-reactant or co-solvent, for a period of time sufficient for the surface modification agent to penetrate the pore structure and produce a nanoporous silica film having a dielectric constant of about 3 or less.
The solution-based processes according to the invention are conducted by reacting the nanoporous silica film with an effective amount of a reaction solution that includes a mixture of an effective amount of a surface modification agent, together with an effective amount of a ketone co-solvent. the reaction is conducted for a period of time sufficient for reaction solution to penetrate the pore structure and produce a nanoporous silica film having a dielectric constant of about 3 or less.
The invention also provides for nanoporous silica films that are hydrophobic and/or that have moisture stable dielectric constants of 3 or less. The invention also provides for integrated circuits that include one or more such nanoporous silica films treated by the vapor phase and liquid phase processes of the invention.
In one aspect of the invention, the surface modification agent employed in the processes of the invention, for both the vapor phase and the solution based processes of the invention, is a compound having one of the following formulas: R3SiNHSiR3, RxSiCly, RxSi(OH)y, R3SiOSiR3, RxSi(OR)y, MpSi(OH)[4xe2x88x92p], RxSi(OCOCH3)y and combinations thereof, wherein x is an integer ranging from 1 to 3, y is an integer ranging from 1 to 3, p is an integer ranging from 2 to 3, each R is an independently selected hydrophobic organic moiety, each M is an independently selected hydrophobic organic moiety; and R and M can be the same or different and are independently organic moieties consisting of alkyl, aryl and/or combinations thereof. Optionally, the aryl is not a heteroaryl group.
In a further aspect of the invention, the alkyl moiety of the surface modification agent, for both the vapor phase and solution based processes of the invention, is substituted or unsubstituted and can be straight alkyl, branched alkyl, cyclic alkyl and combinations thereof, and can range in size from C1 to about C18. Analogously, the aryl moiety is substituted or unsubstituted and can range in size from C5 to about C18.
Thus, simply by way of example, the surface modification agent for both the vapor phase and solution based processes of the invention, is one of the following: trimethylethoxysilane, trimethylmethoxysilane, 2-trimethylsiloxypent-2-ene-4-one, n-(trimethylsilyl)acetamide, 2-(trimethylsilyl) acetic acid, n-(trimethylsilyl)imidazole, trimethylsilylpropiolate, trimethylsilyl(trimethylsiloxy)-acetate, nonamethyltrisilazane, hexamethyldisilazane, hexamethyldisiloxane, trimethylsilanol, triethylsilanol, triphenylsilanol, t-butyldimethylsilanol, diphenylsilanediol, and combinations thereof.
In another preferred aspect of the invention the surface modification agent is an alkylacetoxysilane or arylacetoxysilane compound, e.g., acetoxysilane, acetoxytrimethylsilane, methyltriacetoxysilane, phenyltriacetoxysilane, diacetoxydimethylsilane, diacetoxydiphenylsilane, and combinations of these and/or in combination with any of the foregoing surface modification agents.
In another preferred aspect of the invention, the surface modification agent for both the vapor and solution based processes of the invention is hexamethyldisilazane.
In yet another aspect of the invention, the vapor phase processes of the invention are conducted with one or more co-solvents and/or co-reactants, e.g., including but not limited, ketone co-solvents or co-reactants. Simply by way of example, the co-reactants and co-solvents are selected from among the following compounds: cyclopentanone, diisopropylketone, 2,4- pentanedione, dioxane, n-butanol, 2-pentanol, 1,2-diaminopropane, 1-dimethylamino-2 propanone, water, and combinations thereof.
In a further aspect of the invention, one or more ketone co-solvent(s) for the solution-based processes of the invention are selected from among the following compounds, which are provided simply by way of example: acetone, 2-butanone, 2-pentanone, 3-pentanone, 2,4-dimethyl-3-pentanone, cyclopentanone, cyclohexanone, and combinations thereof